configure: detect and init pkg-config with macro

this also allows to use PKG_CONFIG_* macros immediately
in conditional calls

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>

configure.ac

@@ -94,9 +94,9 @@ AC_PROG_INSTALL
 AC_PROG_LN_S
 AC_PROG_MAKE_SET
 AC_PROG_SED
+PKG_PROG_PKG_CONFIG
 
 AC_CHECK_PROGS([GROFF], [groff])
-AC_CHECK_PROGS([PKGCONFIG], [pkg-config])
 AC_CHECK_PROGS([AUGTOOL], [augtool])
 AC_CHECK_PROGS([DOT], [dot])
 AC_CHECK_PROGS([DOXYGEN], [doxygen])